scook
Files with a .tmp extension are not normally persistent in a project folder. This suggest to me that a file save aborted. This would explain the truncated project file and the existence of a temporary file. I would also recommend against creating .asset files. The files are not used by SONAR anymore. To discontinue writing these files deselect "Save Asset Manifests with Project" in Preferences > File > Advanced
 
 

You are probably right....Sonar has been crashing when closing the application lately...and I often save as I close when it prompts. Probably should avoid that from now!
Thanks for the advice regarding assets.

 I have a hard time imagining that a failed save could produce a project file that opens fine but is missing every bit of user content. I think there must be more to it than that.
 
in any case, another strategy for avoiding problems like this is to save each working session as a new version with the a version number or some note about what you did in the session in the file name. This also avoids saving changes you didn't intend and allows reverting to an earlier version to try a different approach to something. If you had backed up that bad project with the name unchanged, you'd be out of luck.

brundlefly
 I have a hard time imagining that a failed save could produce a project file that opens fine but is missing every bit of user content. I think there must be more to it than that.
 
in any case, another strategy for avoiding problems like this is to save each working session as a new version with the a version number or some note about what you did in the session in the file name. This also avoids saving changes you didn't intend and allows reverting to an earlier version to try a different approach to something. If you had backed up that bad project with the name unchanged, you'd be out of luck.

Unless Sonar simply opens an empty trackview/project by default when something goes wrong (like a landing page)....but that's just guessing.

I think the major reduction in the file size pretty much means that your original project file was somehow hosed. You might try to use a file recovery program to see if you can find the original. Typically a file with the same name will not overwrite an older file automatically in Windows, so a new file may not have irreparably damaged the original. I have never had much luck with file recovery, but if you plan to try it stop using your computer until you do. If you send the current project file to support, I would not expect much help. This is one recovery program that you can try. 
 
https://www.piriform.com/recuva
 

I had the same thing happen to me. What worked for me was finding the previous version. Right click files, properties, previous version. After a couple of minutes there was a list of previous versions. Chose the earliest one before the bad file. Bam, my project was back. I renamed it and saved it again.

Barczar
I had the same thing happen to me. What worked for me was finding the previous version. Right click files, properties, previous version. After a couple of minutes there was a list of previous versions. Chose the earliest one before the bad file. Bam, my project was back. I renamed it and saved it again.

Good (and bad!) to know I'm not the only one :-)